They could make it so PB doesn't effect Farsight AT ALL, Def or Tohit, and it wouldn't break the cottage rule, because PB has not been changed in the slightest.
They could flag EVERY POWER in Time to not allow Power Boost to effect it and it still wouldn't break the cottage rule, because the power in question, Power Boost, still works Exactly the Same.
The cottage rule applies to the power being changed. Would the change to Farsight break the cottage rule for Farsight? No, absolutely not.